This invention generally relates to a magnetic disk drive such as a hard disk drive and, more particularly, it relates to a magnetic recording control system applicable to a magnetic disk drive adapted to realize a high track density and hence a high recording density.
To date, magnetic disk drives such as hard disk drives are popularly used as storage devices in computer systems. The remarkable increase in the processing capacity of computers in recent years has given rise to a strong demand for large capacity HDDs that can cope with the current computer scene where application software is required to provide sophisticated processing capabilities and the volume of data to be processed by a computer is enormously increasing.
The capacity of an HDD can be increased in two ways. Firstly, it can be increased by raising the number of disks and/or the disk diameter that can be dealt with by an HDD. As the number of disks increases, the rotary drive force of the spindle motor for driving the disks should be raised. Then, there arises the problem of increased power consumption rate of the HDD and that of heat and noise generated within the HDD. On the other hand, disks having a large diameter require the use of a large disk drive. Thus, while this method may be feasible for the storage devices of main frame computers, it is not for compact and large capacity HDDs to be used for small computers such as personal computers.
Secondly, the capacity of an HDD can be increased by raising the recording density per disk. The recording density of a disk can be raised by raising the linear recording density and/or the track density. The track density is increased in turn by reducing the track width and the track pitch. The track density refers to the number of tracks per inch as viewed in a radial direction of the disk.
In the case of small HDDs, efforts have been paid to improve the performance of the magnetic head (to be referred to simply as head hereinafter) of HDD for reading/writing data and develop low noise disks in an attempt for improving the recording density. The performance of the head can be improved by reducing the flying height of the head relative to the disk and the gap between the write head and the read head. Additionally, the performance of the head can also be improved by making the upper and lower magnet poles of uniform width to effectively suppress the phenomenon of side writing.
In recent years, composite heads realized by combining a read head and a write head and mounting them on a common slider are in the main stream. A composite head typically comprises an inductive head as write head, although it is conventionally used as read/write head, and a magnetoresistive (MR) head as read head. An MR head is a head that is highly responsive to the existence of a magnetic flux and has an advantage of providing a read back output that is very large if compared with an inductive head and not dependent on the relative velocity of the head and the disk. Giant MRs (GMRS) showing an improved read back relative to MRs are currently under development. As for the write head, research and development efforts are being paid to realize write heads by using a magnet pole material that can generate a high saturated magnetic flux density and a strong recording magnetic field if compared with a conventional core material.
The read head (MR head) and the write head of a composite head can be designed independently. Therefore, it is possible to select an optimal value for the ratio of the effective read back track width, or the width of the read head, to the width of the data track (the effective recording track to be defined by the recording operation of the write head). As described above, the track density can be increased by reducing the track width and the track pitch. However, this technique can raise the rate of appearance of cross talk from adjacently located tracks so that the read head is preferably made narrower than the write head in order to narrow the read back sensitivity distribution across the track. On the other hand, read heads having a width less than 1 .mu.m can be manufactured only by using sophisticated head manufacturing technologies. Particularly, in the case of manufacturing MR heads, complex thin film processes have to be used to minimize the degradation of the magnetic characteristics in the course of forming MR film and improve the dimensional accuracy of the electrodes and hence the read back output.
Generally, in the process of manufacturing heads adapted to a narrow track, a large process tolerance will have to be allowed relative to the track width to consequently lower the yield. In other words, it is currently very difficult to reduce the recording track width in order to improve the track density, while maintaining the ratio of the recording track width (the width of the write head) to the read back track width (the width of the read head), particularly in the case of an MR head that is used as read head.
In view of these circumstances, there is an attempt to allow a relatively high level of cross talk from adjacently located tracks and compensate the negative effect of the high cross talk level by processing signals in a sophisticated manner. This technique normally involves the use of a complicated signal processing algorithm that may require a large volume of hardware and large cost if realized in the form of electronic circuits. Additionally, it cannot completely eliminate the negative effect of a high cross talk level of a magnetic recording system that involves a large number of nonlinear factors. Thus, this technique is not feasible, at least currently, from the viewpoint of cost/performance.
The key for realizing narrow tracks arranged at a small pitch lies in to what extent the cross talk can be suppressed in a magnetic recording system using a write head and a read head having only small widths and to what level the cross talk can be allowed to occur within the system. Particularly, in a system where the read back tracks have a width smaller than 1 .mu.m, the recording track width, the read back track width and the track pitch have to be determined by carefully taking the cross talk from adjacently located tracks into consideration.
Meanwhile, currently available HDDs show a phenomenon that the recording track width changes as a function of the wavelength to be used for recording data. In other words, the longer the wavelength, the greater the recording track width. HDDs are required to meet the performance-related requirements at the smallest track width. In order to design an HDD that operates satisfactorily at the smallest track width, it is highly important to understand to what extent the cross talk from adjacently located tracks affects the data being reproduced as interference noise.
A cross talk is most influential when the longest recording wavelength is found on adjacently located tracks. Therefore, the recording track width, the read back track width and the track pitch have to be so selected as to confine the cross talk level within a permissible range for the system. Then, any data tracks where data are recorded beyond the limited width can baffle the attempt of minimizing the track pitch.
As discussed above, it is difficult with any conventional HDD technologies to meet the requirement of reducing the track pitch and realizing a track width less than 1 .mu.m because of the adverse effect of the cross talk from adjacently located tracks given rise to by fluctuations in the recording track width.